Added a relative mouse mode that uses mouse warping instead of raw input.

To enable this, set the environment variable SDL_MOUSE_RELATIVE_MODE_WARP to "1"

When mouse relative mode is disabled, put the cursor back where the application expects it to be, instead of where it was when relative mode was enabled.

src/video/x11/SDL_x11events.c

@@ -698,7 +698,7 @@ X11_DispatchEvent(_THIS)
 
     case MotionNotify:{
             SDL_Mouse *mouse = SDL_GetMouse();
-            if(!mouse->relative_mode) {
+            if(!mouse->relative_mode || mouse->relative_mode_warp) {
 #ifdef DEBUG_MOTION
                 printf("window %p: X11 motion: %d,%d\n", xevent.xmotion.x, xevent.xmotion.y);
 #endif